Parents often wonder why it takes a year or more and multiple shots to fully immunize their children against diseases like diphtheria and pertussis. The reason is twofold. First, a single vaccination generates only a small amount of immunity and booster shots are needed to build up immunity to protective levels. The second reason is due to the fact that a substantial "lag time" is required by the immune system between initial immunization and subsequent booster shots to maximize the size of...

Word of the Day

pawl

A pivoted catch designed to fall into a notch on a ratchet wheel so as to allow movement in only one direction (e.g. on a windlass or in a clock mechanism), or alternatively to move the wheel in one direction.